Charge transfer layer with hydrazone, acetosol yellow and antioxidant of butylated p-cresol reacted with dicyclopentadiene

ABSTRACT

A photoconductor having a charge transport layer in illustrative embodiments of a polycarbonate binder, 30 to 60 percent by weight DEH hydrazone, 0.5 percent to 5 percent acetosol yellow, and 0.5 percent to 5 percent by weight of the butylated reaction product of p-cresol and dicyclopentadiene. The charge generation layer by type IV oxotitanium phthalocyanine in polyvinylbutyral, poly (methyl-phenyl) siloxane and polyhydroxystyrene. Light fatigue is largely eliminated while the physical and cost advantages of DEH are realized.

RELATED APPLICATIONS

[0001] U.S. patent application Ser. No. 09/237,880; filed Jan. 27, 1999;having some common inventors with this application is directed tocorresponding charge transport layers in which the antioxidant is estercontaining. U.S. patent application Ser. No. 09/584,358; filed May 31,2000; having some common inventors with this application, is directed tocharge generation layer containing the antioxidant of p-cresol reactedwith dicyclopentadiene of this invention. U.S. patent application Ser.No. 09/585,045; filed June 1, 2000, is directed to charge generationlayers having excellent. high-speed operability when the chargetransport layer is consistent with this invention.

FIELD OF THE INVENTION

[0002] The present invention is directed to charge transport layers ofphotoconductors which comprise a hydrazone charge transport compound, aswell as acetosol yellow (also known as SAVINYL YELLOW and Colour IndexSolvent Yellow 138) and an antioxidant.

BACKGROUND OF THE INVENTION

[0003] In electrophotography, a latent image is created on the surfaceof an imaging member which is a photoconducting material by firstuniformly charging the surface and selectively exposing areas of thesurface to light. A difference in electrostatic charge density iscreated between those areas on the surface which are exposed to lightand those areas on the surface which are not exposed to light. Thelatent electrostatic image is developed into a visible image byelectrostatic toners. The toners are selectively attracted to either theexposed or unexposed portions of the photoconductor surface, dependingon the relative electrostatic charges on the photoconductor surface, thedevelopment electrode and the toner.

[0004] Typically, a dual layer electrophotographic photoconductorcomprises a substrate such as a metal ground plane member on which acharge generation layer (CGL) and a charge transport layer (CTL) arecoated. The charge transport layer contains a charge transport materialwhich comprises a hole transport material or an electron transportmaterial. For simplicity, the following discussions herein are directedto the use of a charge transport layer which comprises a hole transportmaterial as the charge transport compound. One skilled in the art willappreciate that if the charge transport layer contains an electrontransport material rather than a hole transport material, the chargeplaced on the photoconductor surface will be opposite that describedherein.

[0005] When the charge transport layer containing a hole transportmaterial is formed on the charge generation layer, a negative charge istypically placed on the photoconductor surface. Conversely, when thecharge generation layer is formed on the charge transport layer, apositive charge is typically placed on the photoconductor surface.Conventionally, the charge generation layer comprises the chargegeneration compound or molecule alone and/or in combination with abinder. The charge transport layer typically comprises a polymericbinder containing the charge transport compound or molecule. The chargegeneration compounds within the charge generation layer are sensitive toimage-forming radiation and photogenerate electron hole pairs therein asa result of absorbing such radiation. The charge transport layer isusually non-absorbent of the image-forming radiation and the chargetransport compounds serve to transport holes to the surface of anegatively charge photoconductor.

[0006] U.S. Pat. No. 4,362,798 to Anderson et al. discloses a layeredelectrophotographic plate or element having a conventional chargegeneration layer and a charge transport layer containing p-typehydrazone and the acetosol yellow of this invention. While thatphotoconductor is particularly good for use in electrophotographyprocesses, it has been found that prolonged exposure to ambient light,and particularly to cool-while fluorescent light usually found inoffices, may decrease the photosensitivity of the photoconductor. Thisis commonly referred to in the art as room light fatigue (RLF). Exposureof such photoconductors to cool-white ambient fluorescent lighting, evenfor just a few minutes, results in a significant shift in the residualvoltage, commonly referred to as fatigue. This shift in residualpotential means that factors such as print density and backgrounddensity will be different on a print made from the fatigued drum whencompared to the last print made before fatiguing this drum. Hence, whena machine is opened for the slightest reason, for example to clear apaper jam, ambient fluorescent light can enter and damage thephotoconductor.

[0007] Typically, room light fatigue does not occur in high speedduplicators, since experienced, well-trained operators commonly servicesuch devices and do not expose the photoconductor to ambient light forprolonged periods. However, room light fatigue typically occurs in lowspeed copiers and printers since such devices are often attended byoperators having little or no training.

[0008] A number of experiments have suggested that one cause of roomlight fatigue is the syn-anti isomerization about the hydrazone C═Ndouble bond. The product acts as a trap and reduces mobility of thecharge transport layer. The preferred hydrazone molecule,p-diethylaminobenzaldehyde-(diphenylhydrazone) (DEH), represented by thestructural formula (I), has been found to experience an undesirablechange in light sensitivity when exposed to conventional cool-whilefluorescent room light for 15 minutes or more.

[0009] The suggestion of a syn-anti isomerization has led to variousapproaches in the art to prevent this isomerization. One of the firstapproaches was the “sunblock” approach. Just as a sunscreen retardslight absorption by human skin pigments, it was suggested thatincorporating a molecule that absorbs at the cool-while fluorescentwavelength would prevent this isomerization. However, large amounts ofthe light-absorbing molecule were typically required in order to absorbmost of the damaging radiation and resulted in a marked decrease inphotosensitivity as charge generation molecule (CGM) and chargetransport molecule (CTM) concentrations were correspondingly reduced.Hence, this was not a viable approach to an RLF-protected, yet fullyfunctional, photoconductor.

[0010] Additional studies in the art have involved the addition of amolecule that could quench the excited singlet state of the hydrazoneCTM, thereby preventing the syn-anti photoisomerization which retardsRLF. However, a need remains for hydrazone-containing photoconductorswhich exhibit reduced room light fatigue.

[0011] U.S. Pat. No. 5,972,549 to Levin et al. discloses photoconductorscomprising a substrate, and a charge transport layer comprising binderand a charge transport compound comprising at least one of hydrazone,aromatic amine or substituted aromatic amine, and a charge generationlayer comprising binder, phthalocyanine charge generating compound, anda hindered hydroxylated aromatic compound. Levin et al. teaches that thehindered hydroxylated aromatic compound reduces electrical fatigue oncycling without adversely affecting the electrical performance of thephotoconuctor.

[0012] Use of DEH as the charge transport material is desirable becauseof its mechanical strength and low cost. DEH containing charge transportlayer provides robustness against handling damage. As operating speedshave increased, crazes and cracks, crystallization of the chargetransport material, and phase separation in the charge transport layerhave been encountered with some CTM's, but DEH does not exhibit suchdefects in charge transport layers.

[0013] Although hydrazones in general show lower mobility than otherknown materials, such as triarylamines, in combination with a certainclass of charge generation materials, as detailed below, excellent,high-speed functioning has been achieved.

SUMMARY OF THE INVENTION

[0014] In accordance with this invention, the charge transport layercomprises a hydrazone charge transport compound, acetosol yellow, and anantioxidant which is the t-butylated reaction product of p-cresol withdicyclopentadiene. (Acetosol yellow is also known as SAVINYL YELLOW andColour Index Solvent Yellow 138.) Photoconductors of this inventioncomprise a substrate, a charge generation layer and the foregoing chargetransport layer. Light fatigue is largely eliminated while the physicaland cost advantages of DEH are realized.

[0015] In certain embodiments having excellent high-speed photoconductorfunctionality, the foregoing charge transport layer is a lamination witha charge generation layer of poly(hydroxystyrene), poly(methyl-phenyl)siloxane, and Type IV phthalocyanine, the poly(hydroxystyrene) being 20percent by weight or less of the total weight of the charge generationlayer.

[0016] In certain embodiments the butylated reaction product is presentin the charge transport layer in an amount of between about 0.5 to about5 percent by weight of the total weight of the charge transport layer.

[0017] In certain embodiments the DEH is present in the charge transportlayer in an amount of between about 30 percent to about 60 percent byweight of the total weight of the charge transport layer.

[0018] In certain embodiments the acetosol yellow is present in thecharge transport layer in an amount of about 0.5 percent to about 5percent by weight of the total weight of the charge transport layer.

DESCRIPTION OF THE EMBODIMENTS

[0019] Photoconductor embodiments described below an anodized and sealedaluminum drum as a conductive substrate, a charge generation layer, anda charge transport layer. The charge generation layer typically iscomprised of a photoconductive pigment, which is dispersed evenly in oneor more types of resin binder before coating. The charge transport layerincludes one or more charge transport molecules, and a resin binder. Theforegoing and all related processing steps may be entirely standard andwidely known, the novelty being in the components employed as described.

[0020] The butylated reaction product of p-cresol and dicyclopentadieneemployed in the charge transport layer is believed to have the followingstructure:

[0021] Wherein n is at least 1, preferably greater than 1, morepreferably from about 1 to about 7. Generally the polymeric hinderedphenol has a molecular weight in the range of from several hundred toabout several thousand, such as from about 460 to about 4600, preferablyfrom about 460 to about 2200. A suitable commercially availablebutylated reaction product of p-cresol and dicyclopentadiene isWingstay® L HLS, available from Goodyear Chemicals.

EXAMPLE 1

[0022] The charge generation layer of these embodiments is the subjectof the foregoing U.S. patent application Ser. No. 09/585,045. Thealuminum drum is coated with a thorough mixture by weight of 45 parts oftype IV oxotitanium phthalocyanine and 55 parts of a blend ofpolyvinylbutyral (PVB), poly(methyl-phenyl) siloxane (PMPSiO) andpolyhydroxystyrene (PHS), in the weight ratio of 50 partspolyvinylbutyral, 45 parts polysiloxane and 5 parts poly(hydroxystyrene)(50/45/5; corresponding ratios of 86/7/7, 90/3/7 and 92/1/7 have verysimilar performance as photoconductors). The coating is from adispersion.

[0023] The polyvinylbutyral is BX-55Z of Sekisui Chemical Company. Thishas the characteristic group of reacted vinylbutyral and also hasethylene alcohol groups.

[0024] The polysiloxane is a standard polysiloxane of commercial purity,specifically Dow Corning 710 Fluid. The backbone of polysiloxanes isalternating silicon and oxygen atoms. Poly(methyl-phenyl)siloxane hasone methyl group substituent and one phenyl group substituent on eachsilicon.

[0025] The polyhydroxystyrene is a standard polymer purchased fromspecifically TriQuest LP.

[0026] The charge transport layer is a thorough blend of a hydrazonecharge transport compound, acetosol yellow, an antioxidant which is thebutylated reaction product of p-cresol with dicyclopentadiene, and apolycarbonate binder coated from a dispersion onto the foregoing chargegeneration layer.

[0027] The polycarbonate binder has the following repeating generalstructure:

[0028] where R₃, R₄ =methyl, cyclohexyl or substituted cyclohexylgroups. When R₃ and R₄ are methyl, the material is polycarbonate A. Inthe following examples the polycarbonate A is MAKROLON 5208 of BayerInc., or, where noted, APEC 9203 also of Bayer Inc.

[0029] In the following examples, the materials were obtained from theaforementioned sources.

Example 2

[0030] (38% DEH; 1% HLS and 1% AY)

[0031] Charge generation layer

[0032] Charge generation (CG) dispersion consists of titanylphthalocyanine (Type IV), polyvinylbutyral, PHS and PMPSiO in a ratio of45/30/15/5 in a mixture of 2-butanone and cyclohexanone. The CGdispersion was dip-coated on aluminum substrates and dried at 100° C.for 15 minutes to give a thickness less than 1 μm, and more preferably,0.2-0.3 μm.

[0033] Charge transport layer

[0034] A charge transport formulation containing 38% DEH was prepared bydissolving DEH (30.7 g), acetosol yellow (0.8 g) Wingstay L HLS (0.8 g)and polycarbonate A (48.5 g), MAKROLON 5208, Bayer Inc. in a mixedsolvent of tetrahydrofuran and cyclopentanone. The charge transportlayer was coated on top of charge generation layer and cured at 100° C.for 1 hour to give a thickness of 24-30 μm.

Example 3

[0035] (36% DE l; 1% HLS and 1% AY)

[0036] Charge generation layer

[0037] Same as in Example 2

[0038] Charge transport layer

[0039] A charge transport formulation containing 36% DEH was prepared bydissolving DEH (29.1 g), acetosol yellow (0.8 g), Wingstay L HLS (0.8 g)and polycarbonate A (50.1 g), in a mixed solvent of tetrahydrofuran andcyclopentanone. The charge transport layer was coated on top of thecharge generation layer and cured at 100° C. for 1 hour to give athickness of 24-30 μm.

Example 4

[0040] (38% DEH; 1% AY, APEC, 1% HLS)

[0041] Charge generation layer

[0042] Same as Example 2

[0043] Charge transport layer

[0044] A charge transport formulation containing 38% DEH was prepared bydissolving DEH (30.7 g), acetosol yellow (0.8 g), APEC 9203 (48.5 g),Wingstay L HLS (0.8 g), in a mixed solvent of tetrahydrofuran andcyclopentanone. Charge transport layer was coated on top of chargegeneration layer and cured at 100° C. for 1 hour to give a thickness of24-30 μm.

Example 5

[0045] (38% DE B; 2% HLS and 1% AY)

[0046] Charge generation layer

[0047] Same as Example 2

[0048] Charge transport layer

[0049] A charge transport formulation containing 38% DEH was prepared bydissolving DEH (30.7 g), acetosol yellow (0.8 g), Wingstay L HLS (1.6 g)and polycarbonate A (47.7 g), in a mixed solvent of tetrahydrofuran andcyclopentanone: The charge transport layer was coated on top of thecharge generation layer and cured at 100° C. for I hour to give athickness of 24-30 μm.

Example 6

[0050] (38% DEH; 3% HLS, 1% AY)

[0051] Charge generation layer

[0052] Same as Example 2

[0053] Charge transport layer

[0054] A charge transport formulation containing 38% DEH was prepared bydissolving DEH (30.7 g), acetosol yellow (0.8 g), Wingstay L HLS (2.4 g)and polycarbonate A (46.9 g), in a mixed solvent of tetrahydrofuran andcyclopentanone. The charge transport layer was coated on top of thecharge generation layer and cured at 100° C. for 1 hour to give athickness of 24-30 μm.

Example 7

[0055] (38% DEH; 5% HLS and 1% AY)

[0056] Charge generation layer

[0057] Same as Example 2

[0058] Charge transport layer

[0059] A charge transport formulation containing 38% DEH was prepared bydissolving DEH (30.7 g), acetosol yellow (0.8 g), Wingstay L HLS (4.0 g)and polycarbonate A (45.3 g), in a mixed solvent of tetrahydrofuran andcyclopentanone. The charge transport layer was coated on top of thecharge generation layer and cured at 100° C. for 1 hour to give athickness of 24-30 μm.

[0060] Embodiments of this invention provide excellent functioning whilepermitting the use of DEH as the charge transport material.

What is claimed is:
 1. A photoconductor comprising a conductive support layer, a charge generation layer, and a charge transport layer, said charge transport layer comprising a binder resin, a hydrazone charge transport material, acetosol yellow and the butylated reaction product of p-cresol and dicyclopentadiene.
 2. The photoconductor as in claim 1 in which said butylated reaction product is present in an amount of between about 0.5 percent to about 5 percent by weight of the total weight of said charge transport layer.
 3. The photoconductor as in claim 1 in which said hydrazone is p-diethylamino benzaldehyde-(diphenylhydrazone) present in an amount of between 30 percent and 60 percent by weight of the total weight of said charge transport layer.
 4. The photoconductor as in claim 2 in which said hydrazone is p-diethylamino benzaldehyde-(diphenylhydrazone) present in an amount of between 30 percent and 60 percent by weight of the total weight of said charge transport layer.
 5. The photoconductor as in claim 2 in which said acetosol yellow is present in an amount between 0.5 percent and 5 percent by weight of the total weight of said charge transport layer.
 6. The photoconductor as in claim 3 in which said acetosol yellow is present in an amount between 0.5 percent and 5 percent by weight of the total weight of said charge transport layer.
 7. The photoconductor as in claim 4 in which said acetosol yellow is present in an amount between 0.5 percent and 5 percent by weight of the total weight of said charge transport layer.
 8. The photoconductor as in claim 7 in which said charge generation layer comprises type IV oxotitanium phthalocyanine, polyvinylbutyral, poly(methyl-phenyl) siloxane and polyhydroxystyrene, said siloxane and said polyhydroxystyrene together making up 20 percent or less by weight of the total weight of said charge generation layer.
 9. The photoconductor as in claim 8 in which said hydrazone is present in amount between about 36 percent and 38 percent by weight of the total weight of said charge transport layer, said acetosol yellow is in an amount of about 1 percent by weight of the total weight of said charge transport layer and said binder resin is polycarbonate. 